The Soret effect arises when a temperature gradient is imposed on a multicomponent system, inducing a concentration gradient. There is no comprehensive theory of the Soret effect that applies to all the systems that have been studied. Polymer electrolytes are a novel and interesting system in which to study the Soret effect due to the dissimilar properties of polymers and salts.
The Soret effect, also known as thermal diffusion, is believed to have been first observed in 1856 by Ludwig, but was not studied comprehensively until 1879, when Charles Soret performed a set of careful experiments on aqueous salt solutions. Each solution was contained in a sealed glass tube with one end in a hot water bath and the other end in a cold water bath. He found that, after sufficient equilibration time, the salt concentration at the cold end of the tube was larger than that at the hot end. This was true for several different salts. Since then this effect has been examined in gas mixtures, liquid solutions, and polymer blends. It has also been studied in colloidal dispersions. However, thermal diffusion has not been studied in mixtures of polymers and salts, which compose polymer electrolytes.
Over the past 150 years several theories have been developed to capture the Soret effect. Each theory has applied reasonably well to a certain class of materials within certain experimental conditions. However, there lacks a universal theory of thermal diffusion that can predict the magnitude of the Soret effect for a given mixture. Even the direction of the Soret coefficient has been difficult to predict. Existing theories on the Soret effect predict that entropy of transport, heat of hydration, or mobility can be used to describe the effect.